Much effort has been devoted in recent years to the development of catalytic converters for removing air pollutants such as hydrocarbons, carbon monoxide and nitrogen oxides from engine exhaust gases. A general approach has been to provide a two-stage conversion system involving an initial contacting zone in which the raw exhaust gases are passed under reducing conditions over a suitable catalyst in the absence of added air, followed by a second zone in which oxidation of remaining CO and hydrocarbons is effected by adding to the first stage off-gases at least a stoichiometric proportion of air. Exemplary reactions which are believed to occur at least to some extent in the first conversion stage are as follows: EQU 2 CO + 2NO .fwdarw. N.sub.2 + 2CO.sub. 2 ( 1) EQU co + h.sub.2 o .revreaction. co.sub.2 + h.sub.2 ( 2) EQU cxHy + xH.sub.2 O .revreaction. xCO + (y/2 + x)H.sub.2 ( 3) EQU 2no + 2h.sub.2 .fwdarw. n.sub.2 + 2h.sub.2 o (4) EQU 2no + 5h.sub.2 .fwdarw. 2nh.sub.3 + 2h.sub.2 o (5) EQU xNO + CxHy .fwdarw. x/2 N.sub.2 + xCO + y/2 H.sub.2 ( 6)
These reactions occur under what may be designated "net reducing" conditions, i.e., conditions wherein the mole ratio of oxygen to carbon monoxide and hydrocarbons is less than stoichiometric. Reactions 2 and 3 seldom go to completion, so that the off gases from the first stage nearly always comprise at least a near equilibrium proportion of carbon monoxide and hydrocarbons. It is hence necessary to provide a second oxidation stage with added air in order to complete the oxidation of carbon monoxide and hydrocarbons. The catalysts of this invention are useful in both stages of these systems, but are exceptionally active for nitrogen oxide (NO.sub.x) conversion under net reducing conditions in the first stage. Moreover, at temperatures above about 900.degree. F., they are very selective for converting NO.sub.x to elemental nitrogen (reactions 1, 4 and 6) rather than to ammonia (reaction 5). This is a decided advantage because any ammonia formed in the first stage is oxidized in the second stage back to NO which is then emitted to the atmosphere as a pollutant.
Several types of catalysts are known in the art which can achieve the desired activity and selectivity for NO.sub.x conversion if the exhaust gases are essentially free of sulfur and lead. However, activity is rapidly lost when one or both of these poisons is introduced into the exhaust gas. Different catalysts exhibit varying responses to each of these poisons. For example, base metal catalysts such as copper-nickel composites are only moderately affected by lead, but are drastically reduced in activity by sulfur and sulfur-lead combinations. Certain noble metals such as platinum and palladium are relatively unaffected by sulfur, but are poisoned significantly by lead, and drastically by lead-sulfur combinations. The search for an economical catalyst which is suitable resistant to both of these poisons has heretofore proven unfruitful.
I have now discovered however that, in sharp contrast to platinum and palladium catalysts, rhodium is remarkably resistant to deactivation by both sulfur and lead, or a combination of the two. Further, whereas platinum and palladium tend to deactivate at high temperatures due to metal agglomeration, rhodium is much less susceptible to this phenomenon and hence gives a more stable catalyst which is highly resistant to thermal deactivation. A fortuitous aspect of the invention is that the rhodium is found to be effective in very small proportions, as little as 0.005 weight-percent on a suitable inert support. Finally, rhodium has an advantage over another lead-resistant noble metal, ruthenium, in that the latter has a much greater propensity to form volatile oxides.
Although rhodium alone is highly active and fairly selective for NO.sub.x conversion to N.sub.2, even greater selectivity is attained by providing an added nickel component, which may either be intimately composited with the rhodium component, or otherwise dispersed in the NO.sub.x conversion zone.
A particularly desirable support material for the active metal or metals is found to comprise certain crystalline aluminum borates.